A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103, which in turn vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The scala tympani forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain. Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104.
In some cases, hearing impairment can be addressed by a cochlear implant that electrically stimulates auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along an implant electrode. FIG. 1 shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processing stage 111 which implements one of various known signal processing schemes. The processed signal is converted by the external signal processing stage 111 into a digital data format, such as a sequence of data frames, for transmission into a receiver processor in an implant housing 108. Besides extracting the audio information, the receiver processor in the implant housing 108 may perform additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through wires in an electrode lead 109 to an implanted electrode array 110. Typically, the electrode array 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
The electrode array 110 penetrates into the cochlea 104 through a surgical opening called a cochleostomy. The electrode array 110 has multiple electrode contacts on or slightly recessed below its outer surface for applying one or more electrical stimulation signals to target audio neural tissue within the cochlea 104. The extra-cochlear electrode lad 109 that goes from the implant housing 108 to the cochleostomy opening usually has no electrical contacts except perhaps a ground electrode and it encloses connecting wires that deliver electrical stimulation signals to the electrode contacts on the electrode array 110.
Insertion and placement and insertion of the electrode array 110 into the cochlea 104 causes trauma to the cochlear tissue due to the rigidity, friction, and impact of moving the electrode array 110 through the cochlea 104. For example, insertion of the electrode array 110 may damage soft tissues, membranes, thin bony shelves, blood vessels, neural elements, etc. In the case of multiple insertions, the damage can accumulate. In addition, removal and replacement of the electrode array 110 due to device failure or aging is also a serious problem. For example, patients with some residual hearing now receive hybrid implant systems that also include acoustic-mechanical stimulation components, and further hearing loss could occur when the electrode array 110 is removed or replaced. In addition, there are efforts to use therapeutic drugs to regrow neural tissue around an inserted electrode array 110 which could suffer catastrophic consequences when the electrode is removed since any new neural tissue growth that might reach the electrode could be disrupted or destroyed.
It has been shown that patients with preserved low frequency hearing have significantly better outcomes than those without such hearing preservation. And it is generally accepted that the amount of electrode insertion trauma correlates significantly with the level of hearing loss caused during the surgery. Thus the extent of hearing preservation preoperatively or postoperatively is believed to serve as a good indicator of the magnitude of electrode insertion trauma. See Skarzynski et al., Atraumatic Round Window Deep Insertion Of Cochlear Electrodes, Acta Otolaryngol. 2011 July; 131(7):740-9. Epub 2011 Apr. 15.
Currently there is no existing developed objective method to evaluate intraoperative hearing trauma. There have been some initial proposals of intraoperative methods to detect possible intraoperative trauma, but all of these approaches still are under development. And all of the existing proposals are based on measuring evoked potentials (e.g., ECAPs) either using acoustic stimuli or some combination of electric and acoustic stimuli, and then recording near-field and/or far-field sensor measurements.
Besides evoked potentials such as ECAPs, measurement of the stapedius reflex response also has been widely used in a clinical practice to evaluate hearing and fit hearing prosthesis systems. The stapedius is the smallest skeletal muscle in the human body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the stapes. The stapedial reflex refers to the involuntary contraction of the stapedius and tensor tympani muscles of the ossicles that occurs in response to a loud sound. The stapedius muscle pulls the stapes (stirrup) of the middle ear away from the oval window of the cochlea and the tensor tympani muscle pulls the malleus (hammer) away from the ear drum. This reflex decreases the transmission of vibrational energy to the cochlea where it is converted into electrical impulses to be processed by the brain for perception as sound.
Several methods to measure the stapedial reflex have been described including:                Recording a pressure change from a probe placed in the ear canal,        Recoding a myogenic response in the vicinity of the stapedial muscle or stapedial tendon, and        Intraoperative surgical observation of the facial nerve during cochlear implant surgery.In combination with other measurements, measurement of the stapedius reflex can be used to determine if a patient suffers with conductive, sensorineural or mixed hearing loss. However, stapedius reflex responses have not been used for monitoring of hearing trauma during cochlear implant surgery.        